The present invention is directed to a combustion control technique and, in particular, for controlling the combustion of fuel, such as refuse in the nature of waste or household garbage, for example, having widely fluctuating thermal values.
A typical combustion plant burns a fuel, and the resulting combustion gases are directed to flow over a heat exchanger forming part of a steam generator. Combustion occurs in a combustion chamber having an opening at one end through which fuel is input and an opening at another end through which ash is removed. The combustion chamber includes a degassing and evaporating zone, a primary combustion zone, and a secondary combustion zone. The degassing and evaporation zone is adjacent the input opening and is, thus, the first zone through which the fuel passes. In this zone, the fuel is subject to heat emanating from the primary combustion zone which is right next to fuel is heated and dried in this first zone, and hydrocarbons are evaporated. Also, some of the fuel may begin to burn. In the primary combustion zone, most of the heat is generated as combustion occurs throughout the fuel located in this zone. All the fuel has ignited, to the extent possible depending on the fuel type, before it leaves this zone In the secondary zone, the burning fuel remains, or dwells, until it is reduced to ash. The heat is passed through a heat exchanger to a steam generator.
Other than in fossil fueled power plants, the control of combustion in a conventional plant such as is described above, but for burning refuse, is typically done manually by an operator. The operator observes the combustion taking place in the combustion chamber, and changes ne or more of the parameters which affect combustion accordingly. Thus, for example, if the combustion has flared to an unacceptable extent, it is damped by reducing the fuel feed rate (i.e. fuel mass flow). If, on the other hand, the combustion has died down below the desired level, the fuel feed rate is increased. Other parameters which affect combustion can also be changed, such as, the volume of primary combustion air supplied from beneath the fuel to each of the three different zones mentioned above, and the relative rate with which fuel is passed through each of the three above-mentioned zones.
It is possible to express the quantity of heat in the combustion chamber with the following equation: EQU Q=M.sub.m .times.H.sub.u (1)
where
Q is heat flow in Kilopond x calories
M.sub.m is mass of fuel in Kiloponds and
H.sub.u is thermal value of the fuel in calories.
Variation of the heat flow is expressed as ##EQU1##
Automatic control of the combustion process has been attempted. This has been done in combustion plants fueled with conventional fossil fuels such as coal, heavy oil, or natural gas which have a relatively uniform thermal value. The steam mass flow from the steam generator is monitored and serves as the controlling parameter for the dH.sub.u /dt term of equation (3) for varying, for example, the fuel and/or air supply rate to effect the requisite change in the combustion process. A certain time delay inevitably occurs between passage in the combustion chamber of the fuel which generates a measured steam mass flow value, and application of the combustion control signal responsive thereto. Nevertheless, such a delay is acceptable because, as mentioned above, the fluctuation in thermal value of such fuels is relatively low. However, for fuels having a highly fluctuating thermal value, such a delay as occurs from using the steam mass flow as the dH.sub.u /dt control parameter is not acceptable. For example, if the fuel is refuse such as household waste and garbage, a bundle of newspaper may be followed by a large mass of garden debris, followed by plastic containers, glass, and so on. These materials have very different relative thermal values. Thus, fuel composed of such materials has a widely fluctuating thermal value. Therefore if the steam mass flow from the steam generator is monitored while the bundle of newspaper is being consumed, for example, it will likely indicate increased combustion due to the high thermal value of paper. Consequently, the control system will tend to reduce the combustion. However, by the time the control system reacts, the newspaper may have been consumed, passed out of the combustion chamber, and the fuel may be grass for which combustion parameters are required to raise the combustion level. As a result, this prior art technique may produce pollutants, incomplete combustion causing the danger of hot, glowing coals being passed to the ash bin, and other disadvantages. Consequently, it is necessary with a fuel having a widely fluctuating thermal value to take the correct value for the dH.sub.u /dt term into account by providing combustion feedback with a faster response time.
It has already been tried to determine the variation of fuel mass density by detecting the air permeability of the fuel as it is passing through the degassing and evaporation zone of the combustion chamber This has been done by measuring the pressure drop in the primary combustion air across the layer of fuel and varying the fuel feed rate such that the average density of the fuel is kept relatively constant. However, the steam mass flow serves in such a technique as the controlling value for dH.sub.u /dt. This approach still has the dual drawbacks of a very long delay or dead time, and the lack of a good, accurate dH.sub.u /dt term.
Because previous efforts to control combustion of fuel with a highly fluctuating thermal value involve a relatively long dead time, complete combustion may not be attained This increases pollution levels, such as carbon monoxide, emanating from the combustion chamber order to keep the ratio of ##EQU2## below acceptable levels, great quantities of air must be introduced into the plant. This requires large and expensive machinery.